Replace bare $var with ${var} everywhere it was still used, matching
the convention already established in rc.d/prompt and rc.d/history.
Positional parameters ($1, $@, $#) and special variables ($?, $$, $-)
are left as-is, consistent with existing usage in _helpers.bash.
rc.d/broot is auto-generated by the broot binary and will likely be
overwritten with its original formatting on the next broot update.
xavier